1. Field of the Invention
The present invention relates to thin film formation using a sputtering target material, a thin film formed by using the same target material and an optical recording medium.
2. Description of the Related Art
Al and Al, alloys have generally been known as material for reflecting film applied to optical recording media including optical discs such as a CD (Compact Disc), a DVD (Digital Versatile Disc) or the like, a magneto optical disc such as a MD (Mini Disc) and MO (Magnetic Optical Disc) or the like or writeable optical recording media such as a phase change optical disc or the like.
Al and Al alloys each can show a reflectivity equal to or higher than a certain value in a specific optical wavelength region when reproducing recorded information with the above described various optical recording media and are excellent in thermal conduction characteristics.
Furthermore, stable coverage can be attained by the Al and Al alloy in grooves with fine recesses and protrusions formed on an optical recording medium and in addition, as an optical recording medium product, excellent resistance to corrosion caused by non-metal elements contained in the air surrounding the product, which leads to an advantage that a change in properties through a long time is very small.
A thin film formed using Al or Al alloys, however, cannot be said to have a sufficiently high reflectivity according to an application of an optical recording medium at present, since a reflectivity of the thin film is on the order of 80% for light with a wavelength of 800 nm, for example.
On the other hand, in a CD-R (Compact Disc-Recordable), when a reflecting film is formed using Al based material, a sufficiently high reflectivity has not been obtained and therefore, application of Au as material of a reflecting film has been studied.
Au, however, has a problem since the element is costly as material of a thin film and therefore, as an alternatives of Au, Ag or Cu has been studied.
Ag, however, has a problem in regard to weatherability in a specific environment such as in sea water or the like since the element Ag is chemically active with non-metal elements such as chlorine, oxygen, sulfur or the like, and ions of the nonmetal elements.
On the other hand, there have been disclosed techniques in the publications of Unexamined Japanese Patent Applications Nos. Sho 57-186244, Hei 7-3363 and Hei 9-156224 in which weatherability is improved by addition of a given impurity to Ag.
That is, in the publication of Unexamined Japanese Patent Application No. Sho 57-186244, a technique is disclosed regarding an Ag—Cu alloy including an Ag content of 40 atomic % or more, in the publication of Unexamined Japanese Patent Application No. Hei 7-3363, a technique is disclosed regarding an Ag—Mg alloy including a Mg content equal to or more than a value in the range of 1 to 10 atomic % and in the publication of Unexamined Japanese Patent Application No. Hei 9-156224, a technique is disclosed regarding an AgOM alloy wherein M indicates Sb, Pd and Pt, an 0 content is in the range of 10 to 40 atomic % and an M content is in the range of 0.1 to 10 atomic %.
In the publications, however, compositions of elements constituting the alloy materials are shown in broad ranges and corrosion resistance and reflectivity of a formed thin film with contents of constituent elements of the alloy materials are not necessarily provided.
Especially, improvement of weatherability by addition of a trace of an impurity to Ag is not achieved at a sufficient level and furthermore, there remain many unclear points on reliability as a reflecting film required for adoption as an optical recording medium.
Besides, Mg is one of the alkaline earth metals and since an element of the kind or its ion is chemically unstable, an alloy prepared using the element has had requirement for improvement of weatherability to chlorine and the like.